I have only passed through Lisbon so I can't speak to that aspect of your question but I have the Sigma 100-300/f4 and have carried it on trips and I'm sure you know how tiring that can get. I have the Tamron 18-250 and can not recommend it highly enough as a travel lens. It may not be the best at max zoom but most travel shots are taken in the midrange and less and it does very well in that area.
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You pay for what you get but you don't necessarily get what you pay for